Insulated containers, also called “coolers,” are prevalent in contemporary life. The insulated containers are often used for picnics or for outdoor activities such as camping or sporting events. In addition, insulated containers are becoming more prevalent in the medical industry, where they are used to move transplant organs and other articles that need to remain cold during transport. Also, the need to transport commercial goods such as perishable food, drink, medicine, and environmental samples is becoming more important.
One downside to current insulated containers is the limited length of time that an insulated container can keep something cold. For example, if ice is used in the insulated container, the ice will often melt because the cooler cannot maintain the colder interior temperatures needed to prevent melting of the ice. Frozen ice packs do not last much longer. Traditional vapor cycle systems, while efficient, are quite large and heavy. Most of these systems require a 110-volt outlet to operate. A few 12 volt or 24 volt systems are available today; however, these systems are also large and heavy. The vapor cycle 12 and 24-volt systems also may have problems with vibrations during transportation. In addition, there exists absorption and adsorption refrigerators, but these fail if enough vibrations exist and improper orientation may also cause the units to fail. Like the vapor cycle refrigerators, these cooler systems are heavy, and must use ammonia in order to freeze.
One solution that has been used for providing insulated containers that can maintain cold temperatures for long periods of time is to incorporate solid state heat pumps such as thermoelectric modules in the insulated containers. Such devices are typically provided power through a DC power input such as a car cigarette lighter adapter.
Many of the newer insulated container refrigeration units utilize a thermoelectric module. Thermoelectric modules are solid state heat pumps based on the Peltier Effect, by which DC current applied across two dissimilar materials causes a temperature differential. A thermoelectric cooler utilizes a thermoelectric module that is capable of providing this temperature differential.
The typical thermoelectric module is manufactured using two thin ceramic wafers with a series of proton (P) and neutron (N) doped bismuth-telluride semiconductor materials sandwiched between them. The ceramic material on both sides of the thermoelectric adds rigidity and electrical insulation. The N type material has an excess of electrons, while the P type material has a deficit of electrons. One P and one N make up a couple. The thermoelectric couples in a thermoelectric module are connected electrically in series and thermally in parallel. A thermoelectric module can contain one to several hundred couples.
As the electrons move from the P type material to the N type material through an electrical connector, the electrons jump to a higher energy state, absorbing thermal energy and providing a cold side of the thermoelectric module. Continuing through the lattice of material, the electrons flow from the N type material to the P type material through an electrical connector, dropping to a lower energy state and releasing energy as heat to the heat sink, providing a hot side of the thermoelectric module.
The fact that a thermoelectric module includes both a hot side and a cold side permits a thermoelectric module to be used to heat or to cool. For insulated containers in which cooling is to be provided, the cold side is used to remove heat from the insulated container.
The fact that a thermoelectric module has a hot side and a cold side presents problems, however. For a thermoelectric insulated container to operate efficiently, the hot side is typically arranged on the outside of the insulated container. However, the cold side needs to be in communication with the interior of the insulated container so that it may remove heat from the interior of the insulated container. Thus, the insulated container must be arranged to effectively allow the hot side and cold side to be mounted in the proper locations. This arrangement usually requires at least a portion of the insulation of a container be removed for the purpose of installing the thermoelectric module and its associated heat transfer components. Removing some of the insulation of the cooler can cause an associated heat loss, which can greatly affect performance of a thermoelectric insulated container.
Another problem with thermoelectric insulated containers is that their assembly is labor intensive. This presents a problem in that labor is expensive. Moreover, because insulated containers are often too large to ship economically, labor for a thermoelectric insulated container often may not be performed at more cost efficient labor areas, such as overseas.